1. Field of the Invention
This invention relates to a sheathed heater, and more particularly to an improvement of a sheathed heater for use at a higher temperature atmosphere.
2. Description of the Prior Art
In the conventional sheathed heater or cartridge heater, a metal pipe, such as a stainless steel pipe is used. Accordingly, if the sheathed heater is used at a high temperature atmosphere of more than 600.degree. C., a leakage current which flows to the metal pipe from the internal heater coil or resistance coil is increased, because the insulation resistance of the heat insulating material member, such as magnesia which is filled in the metal pipe is decreased according to the elevation of the temperature. When the leakage current is increased over a predetermined value, the circuit breaker of the power source for the sheathed heater is activated to break the power circuit. If a plurality of sheathed heaters are used at the same time, the circuit breaker would often be activated to break the power circuit. Thus, the heating operation cannot be carried out effectively.
FIGS. 1 and 2 show a conventional sheathed heater 30 comprising a metal pipe 31, such as stainless steel pipe, a bottom plate 32 covering the metal pipe 31 at a bottom portion thereof, a core of porcelain insulator 33 arranged at the center of the metal pipe 31, a pair of lead pins 34 having base portions which are inserted into small holes formed in and passing through the core of porcelain insulator 33, a heater coil 35 would around the core of porcelain insulator 33, and a heat resistive insulation material member 37 filled in a space formed between the metal pipe 31 and the core of porcelain insulator 33.
In such sheathed heater 30, the insulation resistance of the insulation material member 37 filled in the metal pipe 31 is decreased according to the elevation of the temperature, in case that the sheathed heater is used at a high temperature atmosphere of more than 600.degree. C. Accordingly, a leakage current which flows to the metal pipe 31 from the internal heater coil 35 through the insulation material member 37 is increased so that the circuit breaker (not shown) inserted between the sheathed heater and the power source thereof is activated to break the heating operation. This causes the heating operation to be reduced in efficiency. Further, if a plurality of sheathed heaters are used, the operator must find the sheathed heater the leakage current of which has become large and replace it, and then the circuit breaker must be deactivated with much labor and time.
Further, there are given hitherto manufacturing methods of the sheathed heater as disclosed in Japanese Patents Laid-Open Nos. 155690/83, 157080/83 and 157079/83. In the method shown in the Japanese Patent Laid-Open No. 155690/83, a thin metal plate or metal wire is inserted between the metal pipe and the heater coil. After the sheathed heater is assembled, the metal plate is oxidized in order to prevent the vaporization of the heater coil so that the insulation resistance of the heat insulating material member, such as magnesia, is prevented from being reduced.
However, this plate is incapable of sufficiently reducing the leakage current which flows from the heater coil to the metal pipe at a high temperature atmosphere of more than 600.degree. C.
In the Japanese Patent Laid-Open No. 157080/83, the inner peripheral surface of the metal pipe is coated with a paint including metal oxide powder in order to prevent the vaporization of the device discussed heater coil to attain the same purpose of the device discussed above. However, this arrangement is also incapable of sufficiently inducing the leakage current at a high temperature atmosphere of more than 600.degree. C. It is also very difficult to coat uniformly the entire inner peripheral surface of the metal pipe with the paint. Further, this method is complex and expensive.
In the Japanese Patent Laid-Open No. 157079/83, the inner peripheral surface of the metal pipe is coated with a metal and, after the sheathed heater is assembled, the metal is oxidized with heat to attain the same purpose of the device discussed above. However, in this sheathed heater, the leakage current cannot be reduced sufficiently at a high temperature atmosphere of more than 600.degree. C. as mentioned above.